Remember that Gross had Clear body before mega evolving, and I have a few calcs to keep in mind:
252 Atk Tough Claws Metagross Hammer Arm vs. 88 HP / 0 Def Mega Gyarados: 294-346 (83.2 - 98%) -- guaranteed 2HKO
-1 252 Atk Tough Claws Metagross Hammer Arm vs. 88 HP / 0 Def Mega Gyarados: 194-230 (54.9 - 65.1%) -- guaranteed 2HKO
0 SpA Tough Claws Metagross Grass Knot (120 BP) vs. 88 HP / 0 SpD Mega Gyarados: 206-244 (58.3 - 69.1%) -- guaranteed 2HKO
-1 252 Atk Tough Claws Metagross Thunder Punch vs. 88 HP / 0 Def Gyarados: 384-456 (108.7 - 129.1%) -- guaranteed OHKO
Simply put, MegaShrimp does not beat MegaGross unless you're against AgilityGross or low ladder players, as Hammer Arm is super common on 4atk and I've seen quite a few T-punch and Grass knot ones aswell.
I also support Houndoom for B, It is pretty cool atm and is a great answer to stall for Offensive teams, while not becoming deadweight against other offensive teams because of that lovely base 115 speed. It also has a few niche tools which can come in handy, as Sludge bomb deals with Azu and M-altaria, D-Bond gives you a free kill, HP Ice helps with dragons in general, Flame charge gives you an edge over faster mons and scarfers like Damaged ScarfChomp and Scarf Lando and lets not forget Taunt being Taunt.